Add on Mods

I have successfully added about 9 car add on mods, but now it seems that every new add on car mod i try will not work, i do everything correctly but the game will fail to load, and the only way to load the game is to uninstall the mod. why does every add on car mod doesnt work? also air craft add ons dont work either, can someone help? i tried adding the limitless add on mod, but that also doesnt do anything